1. Field of the Invention
The present invention relates to a machine tool, and more particularly to a machine tool that has a spindle head to/from which a tool can be attached/detached and processes a workpiece by moving the spindle head by a motor.
2. Description of the Related Art
There are some conventional machine tools available which are provided with a tool changer in order to correspond to the weight of a tool and function to change the operating speed of the spindle, because the inertia of the members that exchange, hold and move various tools varies depending on the weight of the tools and influence control of the operation of the machine tools. Owing to such mechanism, these machine tools can be operated at sufficient speed while minimizing the shock thereon or heat generated from the motor, by operating the machine tool at appropriate speed. In such conventional machine tools, usually the operator directly enters the data on the weight of the tool or transfers and inputs the prepared data into the control device of the machine tool.
However, allowing the operator to directly input the tool weight could cause erroneous input. Technology for causing a machine tool to detect the weight of a tool and the like without inputting the weight has been known.
Japanese Patent Application Publication No. H11-188557 discloses a tool changer with a changing arm in which a drive torque necessary for rotating the changing arm gripping a tool is detected based on changes in inertia that are caused by the weight of the tool gripped by the changing arm, and then the weight of the tool is estimated from the drive torque to automatically set the operating speed of the changing arm based on the estimated weight of the tool.
Japanese Patent Application Publication No. H5-138502, on the other hand, discloses a numerical control machining tool that is configured to previously determine the number of allowed rotations of the spindle with respect to a preset inertia moment of the tool, supply a constant power to the drive motor for the spindle after changing the tool, obtain a rise time from the start of the supply of the power to the moment when the rotation speed of the spindle rises to the set value, calculate an inertia moment of the spindle upon installation of the tool based on the obtained rise time, and compare the number of allowed rotations corresponding to the inertia moment with the rotation speed specified by the program, to determine the possibility of the specified rotation speed.
Japanese Patent Application Publication No. 2000-343379 discloses a main spindle head control device that is configured to estimate the weight of the tool on the spindle based on the value of current supplied to the magnetic bearing used in the spindle head, to determine the number of limit rotations of the spindle based on this estimated tool weight.
The rotation speed of the spindle capable of changing tools, the moving speed of the spindle head, and the like are limited in accordance with the weight of each gripped tool. For this reason, it is preferred to input the weight of the tool to be gripped by the spindle and control the weight based on optimal moving speed. It is also preferred to be able to automatically determine the weight of the tool for the purpose of preventing erroneous input of the tool weight.
The arm-type tool changer described in Japanese Patent Application Publication No. H11-188557 detects the drive torque necessary to rotate the changing arm gripping a tool, and then estimates the weight of the tool from this drive torque. However, this technology described in Japanese Patent Application Publication No. H11-188557 can only be applied to a machine tool with an arm-type tool changer.
On the other hand, Japanese Patent Application Publication No. H5-138502 estimates the inertia moment of a tool, thereby exerting the effect of preventing high-speed rotations beyond the capability of the spindle when attached with a heavy tool. However, only the inertia moment of the spindle can be estimated and therefore cannot be utilized in optimizing the tool changing operation or spindle head moving operation in accordance with the tool weight.
The main spindle head control device described in Japanese Patent Application Publication No. 2000-343379 estimates the weight of the tool installed in the spindle based on the value of current supplied to the magnetic bearing. Therefore, this device can only be applied to a machine tool that has a spindle head using a magnetic bearing, and not to a machine tool that does not use a magnetic bearing.